The control of the radial clearance between the tips of rotating blades and the surrounding annular shroud in axial flow gas turbine engines is one known technique for proving engine efficiency. By reducing the blade tip to shroud clearance, designers can reduce the quantity of turbine working fluid which bypasses the blades, thereby increasing engine power output for a given fuel or other engine input.
“Active clearance control” refers to those clearance control arrangements wherein a quantity of working fluid is employed by the clearance control system to regulate the temperature of certain engine structures and thereby control the blade tip to shroud clearance as a result of the thermal expansion or contraction of the cooled structure. It is a feature of such active clearance control systems that the cooling air flow may be switched or modulated responsive to various engine, aircraft, or environmental parameters for causing a reduction in blade tip to shroud clearance during those portions of the engine operating power range wherein such clearance control is most advantageous.
Such active clearance control systems typically consist of multiple parts. These multiple parts not only add weight to the engine structure but also introduce additional parts that may expand and contract under the extreme operating environment. The expansion and contraction of these additional parts must be considered in determining the clearance distance(s) between blade tips and shroud as the additional parts expand and exert inward radial force(s) upon the shroud which could interfere with the blade function.
Consequently, there exists a need for an active clearance control system for gas turbine engines that comprise fewer parts and achieve the desired clearance control aspects without interfering with the function of the gas turbine engine components.